xe2x80x9cThis application is a national phase of PCT/FR99/01457 which was filed on Jun. 17, 1999 and was not published in English.xe2x80x9d
The present invention relates to a method for encapsulating components and for forming a device having one or more components arranged in one or more cavities with a controlled atmosphere.
By cavity, with controlled atmosphere is meant both a cavity in which a vacuum has been set up and a cavity containing a gas with controlled composition and/or pressure.
Said cavity, defined for example by a cover applied to a substrate, can be used to house sensitive components such as electronic, electro-optic or micro-mechanical components.
In particular, the invention finds applications in the hermetic encapsulation of electronic chips, of pressure or acceleration sensors, or further of electromagnetic sensors such as bolometer sensors.
Different techniques are known with which it is possible to bond a cover onto a carrier. For example glass bonding techniques may be cited, or metal-on-metal type bonding, or further anode bonding. In respect of these techniques, well known in themselves, reference may be made to documents (1), (2) and (3) whose references are given at the end of this description.
The bonding of a cover onto a carrier to form a controlled atmosphere cavity is described with reference to FIGS. 1 to 4.
A first operation, illustrated in FIG. 1, consists of positioning a cover 10 on a carrier 12, such as a substrate, in a region containing a component 14.
It is observed that cover 10 has a recess 16 oriented in the direction of the carrier 12 and intended to house the component 14. Recess 16 is surrounded by a border 18.
A strip of insulation 20, in a suitable material such as a fusible material is provided on the surface of carrier 12, so as to surround component 14 and to correspond with the shape of the border 18 in cover 10.
The mutual positioning of cover 10 and carrier 12 is made by alignment means shown very schematically and denoted 22. With these means it is possible to cause the recess 16 of the cover to coincide with the component 14 and to position the border 18 facing the strip of insulation 20.
FIG. 1 shows that all parts to be assembled, and the alignment means, are placed in a chamber 24 in which the desired controlled atmosphere is set up.
The actual bonding step, carried out after positioning of the cover, is illustrated in FIG. 2.
As shown by an arrow, cover 10 is applied to the carrier in such manner that the strip of insulation connects the border 18 in impervious manner to the upper surface of substrate 12.
Optionally, if the strip of insulation is in a fusible material, the carrier and cover assembly may be brought to sufficient temperature to cause the fusible material to melt.
Heating of the cover and carrier, which always takes place in the chamber 24 having a controlled atmosphere, is conducted for example by means of a heating plate 28 on which carrier 12 rests.
FIG. 3 shows, by way of example, a carrier 12 to which three covers 10a, 10b and 10c have been applied.
Reference 10a designates a first cover already bonded to carrier 12. Reference 10b designates a second cover being applied to carrier 12, during the bonding phase.
Finally, a third cover 10c, not yet applied to the carrier, is positioned above a component 14.
It will be noticed that the positioning of the covers and their bonding takes place successively. Also as in the example in FIG. 1, the cover alignment means 22, even though voluminous and cumbersome, must be housed in the chamber 24 with a controlled atmosphere.
The method described with reference to FIG. 3 therefore raises housing problems of the alignment means, and proves to be little adapted to the positioning of a high number of covers onto a carrier.
It is to pointed out that it is not possible with the equipment in FIG. 3 to align and deposit on the carrier all the covers outside chamber 24, and then to conduct the bonding or soldering operation in a controlled atmosphere. For when the cover is placed on the carrier previous to placing the assembly formed by the cover and carrier in a chamber in which a controlled atmosphere is set up, the passage of gas between the cover and the carrier is hindered and it is no longer possible to control precisely the atmosphere which is set up in the cavity formed by the cover and the carrier. This is especially the case when a vacuum is to be set up in the cavity.
One partial solution to the problems mentioned above is provided by a cover transfer system illustrated in FIG. 4.
This system uses an intermediate support part 26 on which covers 10a, 10b, 10c are transferred via their surface which does not come into contact with the final carrier 12.
The intermediate part 26, comprising the covers, is inserted with carrier 12 in a chamber with a controlled atmosphere, and the bonding or sealing of the covers can be conducted collectively.
The method in FIG. 4 nevertheless requires alignment means 22 for the intermediate part 24 with the carrier. Also, the intermediate part must be removed after the covers have been applied.
Another partial solution to the problem of encapsulating a plurality of components, consists of only applying to the carrier a single cover, that is sufficiently large to cover all the components and then to cut the cover individually around each component. This solution, which is not shown in the figures, requires special forming of the carrier and/or cover to form individual cavities around the components, and has recourse to delicate cutting operations.
The purpose of the present invention is to put forward a method for encapsulating one or more components, which does not have the difficulties or constraints described above.
The method is intended to encapsulate components which may either be previously applied to a substrate or directly integrated into the substrate (electronic chips, integrated sensors . . . ).
One purpose of the invention in particular is to provide such a method which may be conducted in a chamber with a controlled atmosphere free of means to align the covers on the components.
A further purpose is to provide such a method with which it is possible to align with precision and to bond collectively a great number of covers over corresponding components.
To attain these purposes, the subject of the invention is more precisely an encapsulation method in a controlled atmosphere of at least one component by bonding at least one cover onto at least one region of a carrier containing the component. In accordance with the method of the invention:
at least one of either the cover or the carrier is provided with bonding means surrounding a region corresponding to said component, and with at least one wedge in a fusible material,
the cover and substrate are mutually positioned so that the cover is arranged substantially opposite a region corresponding to the component,
the assembly formed by the cover and carrier is heated in a chamber with a controlled atmosphere at sufficient temperature to cause the wedge of fusible material to melt.
Also the wedge in fusible material is made with sufficient initial height before fusion to prevent the bonding means from joining the cover to the carrier in impervious manner, and such as to have a height after fusion that is sufficiently low to enable impervious contact of the bonding means both with the cover and with the carrier.
By means of the wedges in fusible material, it is possible to put in place and position the covers on the carrier before inserting the carrier in the controlled atmosphere chamber.
Indeed, the wedges prevent the space between the cover and the carrier from forming a closed, isolated cavity before final bonding. The controlled atmosphere, such as a vacuum for example, may therefore be set up around the component.
Advantage may also be taken of the wedges to serve as electric connections between the carrier and the cover, for example for the transmission of electric signals.
Also, through the method of the invention, the mutual positioning of the cover and carrier, which may be conducted in an ambient atmosphere, is not critical. Since the final, exact positioning of the cover may be obtained automatically through surface tension forces exerting themselves in the fusible material when it is melted.
The method of the invention may be used for a single cover, but proves to be particularly advantageous when a large number of covers must be applied to one same substrate.
According to one particular embodiment of the method, the bonding means may comprise a strip of fusible material able to melt when heated.
The fusible material of the strip is preferably the same material as that of the wedges or a material having a melting point close to that of the wedges.
By way of example, materials such as AuSn, SnPb or CuSn may be chosen. These materials permit hybridisation with no flow of the cover on the carrier.
Therefore, when the cover and carrier are heated, the fusible material melts and forms a sealed joint between the cover and the carrier.
According to one variant, the bonding means may also comprise a strip in another material such as a screen printed glue strip.
Should the bonding means comprise a strip of fusible material, this strip may be designed such as to have an initial height before melting that is less than the initial height of the wedges of fusible material, and a height after meltingxe2x80x94called hybridisation heightxe2x80x94that is greater than the hybridisation height of the wedges.
Such method of proceeding is particularly adapted when the strip and the wedges are arranged in one same plane between the surfaces opposite the cover and the carrier.
By hybridisation height of an element in fusible material is meant the height that this element would have between the cover and the carrier after being melted and in the absence of outside stresses. Therefore, the hybridisation height of the wedges is determined independently from that of the strip and reciprocally.
The hybridisation height of the wedges or of the strip is dependent upon the quantity of fusible material used to produce these elements and on the adhesion surface of these elements to the cover and carrier.
In this respect, it is possible to provide the cover and the carrier with receiver areas for the fusible material that are in a material that is wettable by the fusible material.
The surfaces of the receiver areas may be adjusted in relation to a desired hybridisation height.
The invention also concerns a device comprising a carrier and at least one cover able to be bonded to the carrier to form with the carrier at least one cavity with a controlled atmosphere around at least one component. According to the invention, at least one of the covers or carriers is provided with wedges of fusible material and an insulation strip formed around the component. Also, before bonding, the wedges of fusible material are of sufficient thickness to prevent the insulation strip from coming into contact both with the cover and with the carrier when they are assembled.
Other characteristics and advantages of the present invention will become clearer on reading the following description with reference to the figures of the appended drawings. This description is given solely for illustrative purposes and is non restrictive.